The present invention relates to a method for isolation of casein and calcium phosphate from a milk source.
The recovery of proteins usually requires auxiliary materials to separate and purify the protein. The separation of proteins can be a complex task which may involve several unit operations and consume substantial amounts of auxiliary compounds like acids and bases, that are released as a by-product in the form of salts. In the precipitation of food proteins, this is a considerable environmental burden, due to the large volumes processed.
A number of food proteins, particularly casein and soy protein, are isolated in large volumes by isoelectric precipitation at pH values of 4.5-4.8, using mineral acids such as hydrochloric or sulfuric acid. It is estimated that food protein precipitation processes like this lead to a waste stream of salts of approximately 64 kilotons per year in Europe alone.
In addition to acid precipitation (acid casein) casein can be isolated by enzymatic treatment (rennet casein). Lactic acid fermentation is another well-known option. Organic acids have been used as well, but are nowadays less common. Most of the acid casein is converted to sodium, potassium, magnesium, or ammonium caseinates, depending on the application.
Casein is not a single protein but a group of proteins, comprising four major fractions: xcex1s1 casein, xcex1s2 casein, xcex2 casein, and xcexa casein. They have in common an isoelectric point around pH 4.6. Besides casein, skimmed milk contains a large amount of lactose, the whey proteins among which xcex1-lactalbumin and xcex2-lactoglobulin, certain peptides, salts like calcium phosphate, and several minor components.
Although we are not bound here to a particular theory regarding the structure of a casein micelle, there is a model as described by Walstra and Jenness. Dairy Chemistry and Physics; Marcel Dekker; New York (1994) that is very illustrative. Casein molecules associate into casein micelles, aggregates of 20-200 nm. A schematic representation of the structure is shown in FIG. 1. Within these casein micelles 10 even smaller structures can be identified, so-called sub-micelles 12 (10-20 nm), which are held together by colloidal xe2x80x9cbridgesxe2x80x9d 14 of calcium phosphate connecting phosphatized serine groups in the proteins. Protruding chains 16 are bound to the surfaces of the submicelles 12. Besides calcium phosphate, also other components are involved in these bridges 14, among which are magnesium and citrate. Conversely, calcium is not only bound to the protein via these bridges but also by association with negatively charged groups, along with other cations. During acid precipitation, the calcium phosphate dissolves, as a consequence of which the bridges are broken and the micelle structure is lost. In addition, associated cations are exchanged for protons. For manufacturers of casein, it is important that the dissolution and ion exchange are complete, so that the final product, the caseinate of concern, does not contain residual calcium and phosphate, which strongly influence functional properties, such as emulsifying and water binding properties, and viscosity.
Instead of mineral acids, a volatile acid as carbon dioxide can be used, with the advantage of being easily removed by pressure release. Isoelectric precipitation of proteins with carbon dioxide is thus an alternative to conventional acid precipitation as to reduce the amount of inorganic acids and bases used in recovery processes of food- and biochemicals; like casein. However, also here relatively high calcium concentrations are found in the casein end product (Jordan et al., N.Z.J. Dairy Sci. Technol. 22, 247-256 (1987); Tomasula et al., J. Dairy Sci. 78, 506-514 (1995); and Tomasula et al., J. Food Eng. 33, 405-419 (1997)). Furthermore, the functional properties thereof differ from conventional acid casein (Strange et al., J. Dairy Sci. 81, 1517-1524 (1998)).
Besides casein, another component from milk, calcium phosphate, is a highly appreciated ingredient in many calcium enriched food products, since it is derived from a natural dairy source. It is therefore desirable to also be able to obtain calcium phosphate from milk in a substantially pure form.
In view of the above, it is the object of the present invention to provide a method for preparing casein from a milk source while using a lower amount of inorganic acids and bases than used in conventional casein recovery processes, in which method the casein after isolation does not comprise a considerable amount of calcium phosphate. It is a further object of the invention to provide a means for (re)crystallizing weak electrolytes in general and calcium phosphate in particular, in a controlled manner without adding auxiliary compounds.
The above defined objects are achieved according to the invention by a method for isolating casein and calcium phosphate as separate products from a milk source, comprising the steps of:
a) contacting the milk source with carbon dioxide under pressure in order to precipitate the casein;
b) separating the casein from the milk source while maintaining the pressure to obtain a casein fraction and a whey fraction;
c) releasing the pressure from the whey fraction to allow calcium phosphate to precipitate therefrom.